1. Field of the Invention
The present invention relates to apogee detection for projectiles such as mortar rounds and rockets, and more specifically to apogee detection circuits for launched or fired ballistic projectiles. The present invention further relates to apogee detection in order to transform and/or guide projectiles. The present invention further relates to methods of detecting apogee. The present invention further relates to a methods of triggering arming of a projectile fuze, ignition of an on-board propulsion system, deployment or activation of control surfaces or flow effectors, activation of power-consumptive sensors or systems, initiation of range improvement or precision targeting systems, morphing or transformation of the projectile airbody including by the shedding of surfaces or release or exposure of a payload, and similar changes related to a new phase of projectile flight. The present invention further relates to methods of guiding glidable and/or steerable ballistic projectiles using guidance systems activated and powered only after an appropriate time based at least in part on the detection and/or prediction of apogee, including “man-in-the-loop” human guidance by radio-transmitted video signals and guidance signals.
2. Terminology and Technology Review
Ballistic apogee is the peak altitude (highest point) in an arcing projectile's trajectory, i.e., the roughly midflight point at which vertical velocity reaches zero. Apogee detection for projectiles such as munitions and rockets is useful for triggering an apogee-related event. In some cases this event may be the deployment of a drogue parachute to begin a descent phase and bring a rocket safely back to the ground, or the triggering of an arming or detonation process in the fuze of a weaponized projectile. In more advanced applications such as those to which the present invention may be adapted, apogee (or some time related to apogee) may trigger some other change related to a new phase of projectile flight, such as ignition of an on-board propulsion system, a deployment or morphing of control surfaces, an initiation of range improvement or precision targeting systems, a transformation of the projectile including by the shedding of surfaces or release of a payload, etc. As such, inaccurate apogee detection can lead to mistiming and failure of a mission-critical action. Thus, accurate detection—and, in some applications, accurate advance prediction—of ballistic apogee is a useful aim and still an unsolved problem for many of the challenges it presents.
The simplest type of apogee detector may use some mechanical mechanism to detect nose-over. A conductive ball bearing set loosely in a chamber, for example, may shift forward upon nose-over to complete a circuit and send a signal indicating that apogee has been reached. However, such a mechanism is unreliable, given that it may send (1) a false-positive indication of apogee if the projectile is buffeted in flight or the ball bearing is otherwise jostled into a circuit-completing position, or (2) a false-negative indication of apogee if the ball bearing (or other mechanical mechanism) becomes stuck or otherwise fails, possibly as the result of damage from g forces experienced on launch. Furthermore, such a mechanism may only detect apogee once it has already occurred, and cannot predict apogee, so as to signal the triggering of some event that should occur or begin some defined time prior to apogee. Given the drawbacks of purely mechanical apogee detectors, such devices should not be used for mission-critical applications.
Other apogee detectors are altimeter-based. Traditional projectile altimeters use one or more barometric sensors, or barometric sensors combined with accelerometric sensors, to determine altitude. Calculations are performed on the outputs of such sensor(s) to generate an estimate of the dynamic state. The accuracy of the estimate, and thus the usefulness of the altimeter, is improved by reducing and/or compensating for error that results from sensor measurement noise.
A barometric altimeter, for example, determines altitude by measuring the air pressure inside the projectile's electronics bay as a proxy for external static air pressure, and applying the atmospheric standard model to that measurement. The use of barometric altimeters have numerous disadvantages. For one, a fuze system using a barometric altimeter to detect apogee must be “air-breathing” in order to utilize pressure sensors. Permitting a barometric altimeter access to air adds costs from parts and routing, and presents undesirable aerodynamics design challenges because an airframe hole needed to supply air to a pressure sensor can create a vacuum that may need to be compensated for by the airframe design and can result in excess drag. Such “air-breathing” designs limit the altitude, velocity and robustness of the fuze system, as pressure sensors are highly sensitive to these parameters. Another downside results from the fact that the proxy measurement may not exactly match the actual external static air pressure and thus the resulting estimation may not always provide sufficient accuracy. Resultantly, apogee is generally “detected” at some minimum offset pressure from (peak altitude) minimum pressure, in order to ensure that a false apogee detection is not triggered by measurement system noise on ascent. Resultantly, such a system “detects” apogee later than apogee actually occurs. Filtering the sensor output to reduce noise likewise results in loss of response and apogee detection delay. Heuristic rules used to deal with onboard sensor noise or error, such as the application of mach delays with barometric altimeters to compensate for pressure errors as a projectile approaches and passes through the speed of sound, may further improve apogee detection, but heuristic approaches require anticipation of possible errors and creation of compensation rules. Given the various drawbacks of pressure-based approaches to detecting apogee, it would be desirable to be able to eliminate any barometric sensors from an apogee detector design.
In apogee detector designs that use accelerometers, the apogee detection works by integrating measured acceleration to find velocity, and pronouncing apogee detection at some threshold low vertical velocity. This approach has not been found to be as reliable as barometric-based approaches, and the integration process is computationally slow and accumulates error with each integration step. As with apogee detection based on barometric pressure, measurement noise reduces the accuracy of the apogee determination. Detection of apogee with an accelerometric measurement alone may not provide sufficient reliability for mission-critical applications, particularly in high-g scenarios.
Given noisy sensor measurements, a least-squares filtering method may improve trajectory estimation, but disadvantageously involves greater data storage and computation requirements. Usage of a Kalman filter, by contrast, provides a similar estimation fitted to a dynamic model instead of an arbitrary polynomial, and requires less storage of data history and less computational intensity because it is recursive (only the previous estimate is needed to calculate the next estimate) and a curve fit is not needed at each measurement.
As a result, apogee detectors that use accelerometry sensors generally also use air pressure sensors and rely at least in part on barometric pressure readings to detect apogee.
Existing apogee detection devices are also larger and heavier than desirable. Size and weight are important considerations for a component intended to be placed into a projectile, where free space is scarce and excess weight increases fuel/charge requirements or reduces range.
In view of the foregoing disadvantages of present apogee detection devices and methods, what is needed is an apogee detection system that does not rely on external environmental data to detect apogee, but instead provides an entirely self-contained modular system that relies on the projectile's attitude and motion to predict and detect apogee. Further what is needed is an apogee detection system that is capable of fusing data from multiple sensors to provide accurate state and orientation information about the projectile and thus to provide accurate prediction of the ballistic apogee of the projectile. Further what is needed is an apogee detector that does not rely on external environmental information or a priori knowledge to make the apogee prediction or detection. Further what is needed is an apogee detector that is computationally efficient, fast, and accurate. Further what is needed is an apogee detector that is small and lightweight. Further what is needed is an apogee detector that can easily be installed and replaced in a fuze or other part of a projectile with a minimum of connections/disconnections. Further what is needed is an apogee detector that can not only detect but also predict apogee and thus permit the projectile or fuze to initiate any number of mission processes, including but not limited to fuze arming, targeting control, airbody transformation, maneuvering, flow effector deployment or activation, and/or payload exposure or deployment.